I. Field of the Invention
The present invention generally relates to wireless telephones. More specifically, the present invention relates to techniques involving the automatic adjustment of RF amplification circuitry.
I. Description of the Related Art
Signals transmitted by wireless telephones are required to satisfy various requirements. For instance, Code Division Multiple Access (CDMA) cellular phones are mandated by the FCC to limit out of channel distortion when transmitting in the radio frequency (RF) spectrum. Adjacent Channel Power Rejection (ACPR) is a metric frequently used to measure out of channel distortion. ACPR is represented as a curve across the spectrum that is centered at a transmitted RF signal""s center frequency. At this center frequency, an ACPR curve is at its maximum. However, an ACPR curve symmetrically attenuates as frequencies depart from this center frequency. ACPR curves are compared against the spectral power characteristics of transmitted RF signals. Current CDMA standards, such as IS-98, require the spectral power characteristics of transmitted CDMA signals to be below a defined maximum ACPR curve at all frequencies and transmit power levels. When a signal complies with such a requirement, the signal is said to have passing margin. When a signal fails to comply with such a requirement, its out of channel distortion is excessive.
A wireless phone contains components that amplify RF signals so that they have sufficient power for transmission. Before amplification, a properly modulated RF signal has negligible out of channel distortion. An amplified signal""s out of channel distortion will also be negligible if the amplification process is linear. However, if a signal has been amplified by a non-linear amplification process, its spectrum will include increased out of channel distortion. This increased out of channel distortion may cause a wireless phone to exceed the maximum allowed ACPR.
Electronic amplifiers are generally linear devices. However, under certain conditions, amplifiers will behave in a non-linear fashion. These conditions include low supply voltage and high temperature. Non-linear performance can be reduced by adjusting the output power produced through amplification. This reduction of non-linear performance will also reduce out of channel distortion. What is needed is a way to monitor operating conditions to provide the maximum possible output power without surpassing specified ACPR limits.
The present invention is a method and system for maintaining adjacent channel power rejection (ACPR) passing margin. The method and system involves the control of an automatic gain control (AGC) amplifier to achieve a power amplifier (PA) output power that is appropriate for the operating conditions.
A method of the present invention includes amplifying a first radio frequency (RF) signal according to a first gain to produce a second RF signal and amplifying the second RF signal according to a second gain to produce a third RF signal. The method also includes determining a desired power level of the third RF signal, computing a new gain value from the desired power level, and adjusting the first gain to the new value.
A system of the present invention includes an automatic gain control (AGC) amplifier having an AGC input terminal, an AGC output terminal, and a control signal input terminal. The system also includes a power amplifier (PA) having a PA input terminal and a PA output terminal, wherein the PA input terminal is connected to the AGC output terminal. In addition, the system includes an output power controller having a controller input terminal and a controller output terminal, wherein the controller input terminal is connected to the PA output terminal and the controller output terminal is connected to the AGC input terminal.
An advantage of the present invention is the maintenance of ACPR passing margin throughout a range of operating voltages and temperatures without unduly compromising output power.